Always making a bigger threat in the next film is not what makes for a compelling story. Creating a bigger bad in the following film puts an unnecessary burden on the sequel that it will eventually fail to carry through.

Red Skull was a big threat to the world that Winter Soldier is unlikely to match but Winter Soldier is much more personal to Cap. If we are going on an escalating threat level, it seems to me Winter Soldier should have come before Red Skull, but that's not how the series is working. The Cap/Bucky relationship is what will make the film work, not the threat to the world.

I also thought Joker was a smaller threat, very personal but smaller, than Al Ghul was but that could just be me.